1. Field of the Invention
This invention relates to sonar sources, in particular active sonar sources, and especially to multifrequency acoustic projectors, having transmit frequency bands which are widely separated in the frequency domain.
2. Description of the Prior Art
In general, generating active sonar signals in multiple frequency bands is of interest for two reasons. First, different types of objects of interest, or targets, need to be insonified in different frequency regimes. Second, sonar systems may be required to operate in shallow water and deep ocean environments, both of which present different, frequency-dependent signal propagation problems. Current practice in the design of acoustic projectors which are capable of transmitting in multiple frequency bands requires the design and construction of separate acoustic projectors for each desired transmit frequency, and the assembly of the individual projectors into an acoustic projector array. The number of different acoustic projector designs in the array usually equals the number of desired frequency bands for signal generation.
For example, in a two-frequency-band array, some fraction of the array projectors is designed to resonate at one common frequency while the remainder of projectors in the array is composed of acoustic projectors which are designed to resonate at a frequency different from that of the first fraction of the array.
This approach is hardware-intensive and may result from current acoustic projector architectures which typically possess only a single, in-water, volumetric mode resonance frequency. Another description for a volumetric mode is a "breathing" mode which is excited at its fundamental frequency. Examples of such architectures include Tonpilz, bender bars, baffled and unbaffled vibrating pistons, baffled and unbaffled flexural disks, flooded rings, and split cylinders. For an example of an acoustic projector with a single volumetric mode shape using a split cylinder transducer, see U.S. Pat. No. 5,020,035 issued to Kompanek.